In oil exploration after a well has been drilled, various well logs are taken to determine porosity of the well and the location of oil or hydrocarbon zones. Spontaneous potential (SP) logs are taken to determine porous zones. Resistivity logs are taken to determine oil or hydrocarbon zones within the porous zones. Normally, hydrocarbon and water interfaces are determined through the resistivity logs since oil and gas have appreciably higher resistivity than that of formation waters. A formation mineral pyrite (FeS.sub.2) has a resistivity similar to that of formation water. When pyrite is present in a hydrocarbon zone, a resistivity log may give an erroneously low indication of hydrocarbon content.
A prior art method used to distinguish hydrocarbon zones having pyrite from water zones has been to take an additional log, a density log. Pyrite has a significantly higher density than formation water and the density log can be used to determine whether a low resistivity porous zone is formation water or hydrocarbons with pyrite. However, when an oil zone is near a pyrite zone, siderite (FeCO.sub.3) may also be present. Siderite is essentially nonconductive and presents no false readings on a resistivity log, however siderite has a high density like pyrite. A hydrocarbon zone containing pyrite and a water zone containing siderite may have similar resistivity and density log responses. Thus, when pyrite and siderite are present in water and hydrocarbon bearing zones a combination of density and resistivity logs cannot accurately identify water/oil content.